1. Field of the Invention
The present invention relates to the field of metal matrix composite materials and its subject is more particularly a method for manufacturing an insert formed of ceramic fibres in a metal matrix for reinforcing a metal part.
2. Description of the Related Art
Particularly in the aeronautics field, a constant aim is to optimize the strength of the parts for a minimal weight and footprint. Certain parts may now comprise an insert made of metal matrix composite material hereinafter called CMM, the part also being able to be monolithic. Such a composite material comprises a metal alloy matrix, for example of titanium alloy Ti, within which fibres extend, for example silicon carbide SiC ceramic fibres. Such fibres have a tensile strength that is markedly greater than that of titanium (typically 4000 MPa as opposed to 1000 MPa). It is therefore the fibres that absorb the forces, the metal alloy matrix performing a binder function with the rest of the part, and of protection and of isolation of the fibres, which must not come into contact with one another. Moreover, the ceramic fibres are erosion resistant but must necessarily be reinforced by metal.
These composite materials can be used in the manufacture of discs, of shafts, of cylinder bodies, of housings, of struts, such as reinforcements for monolithic parts such as blades, etc.
For compressor discs in a turbojet for example, a known reinforcement technique consists in inserting into the part a circular winding of coated fibres. One technique for manufacturing a CMM insert relies on the principle of winding coated yarns described in patent EP 1.726.677 filed in the name of Snecma. The insert is obtained from a plurality of coated yarns each comprising a ceramic fibre covered with a metal sheath. This type of yarn is called a coated yarn hereinafter. The manufacture comprises a step of winding a bundle or a bound layer of coated yarns around a part of revolution perpendicularly to the axis of the part. The insert is then subjected to a step of hot isostatic compression in a container. A container is a metal part blank into which a cavity has been machined for receiving the insert made of CMM material and which is then subjected to a hot isostatic compaction treatment. This treatment is called CIC hereinafter.
The parts described thus obtained are of circular type and are particularly suitable for not only compressor discs but for the production of circular parts such as shafts, cylinder bodies or housings.
Other mechanical parts require properties that differ from those presented by the circular parts. This is particularly the case of the connecting rods used for example in landing systems or of the structural parts such as engine suspensions, that are essentially of oblong shape. The function of these parts is to transmit a one-way traction and/or compression force. The reinforcement of these parts then requires inserts made of CMM that are rectilinear or substantially rectilinear in shape, at least in part. Specifically, the fibres must be oriented in the direction of the forces.
The manufacture of these inserts in an industrial manner and at least cost is awkward.
A method is known for manufacturing a mechanical part comprising at least one insert made of CMM material. The method comprises the manufacture of an insert blank by winding a bundle or a bound layer of coated yarns about an annular support of which a portion comprises a rectilinear or substantially rectilinear section.
The method described in patent FR 2.919.284 in the names of Snecma and Messier-Dowty develops this principle and then comprises the insertion of the insert blank hereinabove in a first metal container, the hot isostatic compaction of the first container, followed by the machining of the latter in order to form an insert element. After the manufacture of this insert element, the method for manufacturing a mechanical part comprises the following steps: insertion of the insert element into a second container, hot isostatic compaction of the second container and machining of the second container in order to form the desired mechanical part. The mechanical part thus obtained, for example a connecting rod, advantageously makes it possible to transmit one-way traction and/or compression forces in the direction of the ceramic fibres that have been incorporated therein.
Instead of passing through the intermediate step of compaction of the insert blank followed by cutting it into rectilinear insert elements, it would be possible to envisage cutting the annular coil forming the blank while keeping the coated yarns in a bundle. Patent application FR 2.925.896 teaches of the incorporation of this type of bundle in a rectilinear groove opening out at its ends.
This solution has several drawbacks which have an effect on the industrialization of these operations:
The coated yarns are lost in the non-straight portions. This loss is not insignificant because the half-produced coated yarn represents a considerable cost in the total cost of the part.
The winding, notably on oval shapes, induces stresses in the wound insert that risk being released which results in a deformation of the insert when cut.
These techniques require increasing the number of systems for keeping the coated yarns in line with the cutting zones.
Moreover, a technique based on coiling by winding a layer of previously assembled coated yarns is aimed essentially at the production of inserts with a cross section, perpendicular to the fibres, that is square or rectangular.
For certain applications, it would be desirable to have an insert with a section that is not square or rectangular in order to improve the absorption of forces between the composite insert and the rest of the structure of the part. Specifically, reinforcements with a cross section that is for example trapezoidal or elliptical would make it possible to prevent or at least to limit the leaps in stiffness and thus improve the mechanical strength of the transition zones.
For example, for elongate parts such as parts of landing gear, or connecting rods for suspending the engine comprising lateral attachments between their ends, a reinforcement in which the number of fibres is smaller along the edge of the part secured to the attachment allows a better transition of the forces at this attachment.